Device for reflecting and detecting electromagnetic radiation

ABSTRACT

A device for simultaneously detecting and reflecting electromagnetic radiation consisting of a thin layer of insulating pyro-electric and/or piezoelectric material sandwiched between two conducting electrodes. The upper-most electrode is effective to separate the radiation into a reflected part and an unreflected part, which is absorbed, and the insulating layer has an electrical property dependent on the intensity of electromagnetic radiation absorbed by the upper-most electrode. An electrical voltage and/or current measured between the two electrodes is responsive to the electrical property of the insulating layer and is indicative of the intensity of the absorbed electromagnetic radiation.

This invention relates to a device for reflecting and detecting incidentelectromagnetic radiation.

Conventional detectors of electromagnetic radiation are designed andoptimised to absorb and provide an output representative of the totalelectromagnetic radiation incident on their active area. Such detectorsare accurately described as endpoint detectors, and are intended tomeasure the intensity of light at the end of an optical path in aninstrument or analyser. These devices can be used to detect pulsedelectromagnetic radiation or continuous wave (CW) electromagneticradiation.

An alternative type of device is used when it is required to measure apulse of light from a laser at a number of points along the optical pathof the light, providing a type of multiple intermediate point detectionsystem. Currently available devices comprise distinct components;namely, a mirror comprising a highly reflective surface on anelectromagnetic radiation transmissive substrate, and a separatedetector. The detector measures the energy of the electromagneticradiation which is not reflected by the mirror but passes through themirror. This type of device is used for the sensitive analyticaltechnique of Cavity Ring Down (CRD) Spectroscopy. In this analyticaltechnique a single small area detector (˜1-20 mm²) is used, and pulsesof light are periodically returned to the detector by use of an opticalcavity.

According to the invention there is provided a device for simultaneouslyreflecting and detecting electromagnetic radiation, comprising a firstlayer made from electrically conductive material for simultaneouslyreflecting and absorbing electromagnetic radiation incident at a surfaceof the layer, wherein said first layer simultaneously separates incidentelectromagnetic radiation into a reflected part and an unreflected part,the first layer being effective to reflect the electromagnetic radiationof said reflected part away from the device and to absorb theelectromagnetic radiation of the unreflected part, a second layerunderlying said first layer, made from a material having an electricalproperty dependent on an intensity of electromagnetic radiation absorbedby said first layer, and a third layer underlying said second layer,made from electrically conductive material, wherein said first layer andsaid second layer form a first electrode and a second electroderespectively and electrical voltage and/or current measured between theelectrodes is responsive to said electrical property and indicative ofthe intensity of the absorbed electromagnetic radiation.

The detection surface, in effect, defines a combined mirror and detector(a detecting mirror). The aim is to reflect a known proportion of theincident electromagnetic radiation whilst efficiently measuring theunreflected part of the incident electromagnetic radiation. An advantageof this device compared with devices known from the prior art is that itremoves the device substrate from the optical path. This advantage isparticularly important when detecting radiation in the infra-red regionof the electromagnetic spectrum, where transmissive componentsfrequently present unavoidable compromises between optical andmechanical parameters that can lead to signifcantly reduced performancefrom that of the ideal.

A device according to the invention can have a fast responsecharacteristic and so is well suited to the detection of short pulselaser signals and ring down signals.

The invention finds particular, through not exclusive application in thedetection of infra-red radiation. The use of infra-red radiation forspectroscopic applications is desirable since absorption bands are notonly stronger but also have less complex structures. This enablessensitive measurements to be made with far less spectral ambiguity.

Many prior art detectors suitable for use in the infra-red region of theelectromagnetic spectrum generally do not have fast time responsecharacteristics, and those that do typically only have a small activearea (1-20 mm²) for detection of the radiation.

By contrast, a device according to the invention may have a large activearea (typically 500 mm²) and yet may still achieve sub-nanosecondresponse characteristics (typically 0.5 to 2 nanoseconds). This is animportant feature which cannot be achieved in a cost effective mannerusing conventional devices. The time response characteristics and thesensitivity of the device are also uniform over the whole of the activearea, and the device has a high level of physical and opticalrobustness.

The absorption process typically takes place in the first layer on afemtosecond timescale and so the sub-nanosecond response time of thedevice is principally dependent on the response time of the insulatingmaterial of the second layer. Typically, insulating materials such asPVDF or the co-polymer PVDF/TrFE are used, although other materialshaving faster response times could alternatively be used.

As already described, existing devices comprise a number of distinctcomponent parts, whereas, by contrast, a device according to theinvention is a singular device having an integrated structure.

An embodiment of the invention is now described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 shows a transverse cross-sectional view through a device fordetecting and reflecting electromagnetic radiation according to theinvention,

FIG. 2 shows a top view of the device shown in FIG. 1 and

FIG. 3 shows a bottom view of the device shown in FIG. 1.

FIG. 1 shows a fast-responding large active area electromagneticradiation detection device particularly suitable for detecting andreflecting short laser pulses. The device is constructed from a pair ofmetal electrodes 1,2 disposed to either side of a thin layer 3 of apyro-electrically and/or piezo-electrically active insulating material.The electrodes 1,2 and insulating layer 3 are mounted on a preformedsubstrate 4. This substrate is preformed to have a desired shape towhich the electrodes 1, 2 and layer 3 conform. The substrate istypically mounted directly on the front of a printed circuit board (PCB)5 and pre-amplifier electronics 6 are mounted on the rear of the PCB. Ifdesired, the device can be mounted inside a screened can (not shown) tominimise exposure to externally generated radio-frequency (RF)interference.

The upper-most metal electrode 1 provides a surface on which theelectromagnetic radiation is incident. The electrode 1 is made from athin layer of optically opaque and electrically conductive material.This layer performs several different functions in the device.

Firstly, the layer has a specularly reflective surface i.e. it obeysSnell's law of reflection, and so acts as a mirror reflecting aproportion of the incident electromagnetic radiation. To this end, theuppermost surface of the layer has a desired optical flatness. Asalready explained, the shape of the mirror is determined by the shape ofsubstrate 4 which has a desired optical finish. In this example themirror is concave. The layer also absorbs the energy of theelectromagnetic radiation which is not reflected and transmits theabsorbed energy to the insulating layer 3. Finally, the layer acts as anelectrode which, in association with electrode 2, allows a current orvoltage output generated across the insulating layer 3 to be measured.Alternatively, this first layer may have a diffusively reflectivesurface.

The material used for the upper-most metal electrode 1 is chosenforemost for its optical and electrical properties but may also bechosen for its chemical properties as well. Typically, the metalelectrode 1 is made from silver, gold, aluminium or copper but othermetals can alternatively be used. This upper-most metal electrode 1 mayhave an additional layer 10 provided as a coating on its top surface.This layer 10 may be transparent to a particular range of wavelength andcan act as a high, low or band pass filter. Alternatively, oradditionally, layer 10 may act as a chemically protective layer, forexample to protect the metal layer 1 from oxidation. This additionallayer 10 may be particularly reflective to one or more band ofwavelengths, and optically transmissive to all other wavelengths. Inthis case the additional layer 10 can be used to provide effectiveattenuation of the intensity of electromagnetic radiation reaching theupper-most electrode 1, enabling the reflectivity/absorption ratio ofthe upper-most electrode 1 to be finely controlled. This additionallayer 10 may be a single layer or it may be comprised of two or morelayers. The additional layer 10 preferably conforms to the shape ofmetal electrode 1, but other shapes are contemplated.

The minimum permissible thickness for the upper-most metal electrode 1is defined by electrical conductivity and optical opacity requirementsof the device, and the maximum thickness of the upper-most metalelectrode 1 is defined by the need to suppress unwanted mechanical andelectromechanical resonances. Typically, electrode 1 has a uniformthickness between 0.5 μm and 100 μm, but other thicknesses outside thisrange may be used.

In preferred embodiments the upper-most electrode 1 is formed bydepositing a continuous uniform metal film on the piezo and/orpyro-electrically active insulating layer 3.

The insulating layer 3 is provided between, and separates the two metalelectrodes 1,2 and also acts as a detection medium for the energyabsorbed by the upper-most metal electrode 1. The absorbed energy isdetected by monitoring the pyroelectric and dielectric properties of theinsulating layer 3. More specifically, an electrical property of thematerial of the insulating layer 3 depends on the intensity ofelectromagnetic radiation absorbed by the material of electrode 1, sothat electrical voltage and/or current measured across the insulatinglayer, between electrodes 1,2 will be indicative of the intensity ofelectromagnetic radiation in the unreflected part of the incidentradiation. It is thought that absorption of electromagnetic radiation bythe electrically conductive material of layer 1 causes a change in thepolarization and dielectric property of the piezo and/orpyro-electrically active material creating measurable charge atelectrodes 1,2. The insulating layer 3 is typically made from apyro-electrically and piezo-electrically active polymer film such aspoly(vinylidene difluoride) (PVDF) or the copolymer of poly(vinylidenedifluoride)/trifluoroethylene (PVDF/TrFE). Such materials require polingbefore becoming pyro-electrically and piezo-electrically active. Themethods and techniques for carrying outthis procedure are well known andexamples are described in Miranda el al Appl. Phys. A 50 p431-438(1990).

The lower-most electrode 2 is also made from electrically conductivematerial and provides a second output electrode. This second electrode 2may be a thin metallic layer (0.5-100 μm) deposited on the insulatingsubstrate 4, or alternatively it may comprise the substrate 4.

The device is electrically terminated to take account of therequirements of transmission lines and output impedance suitable forhigh frequency and ultra-high frequency operation, and pre-amplifierelectronics 6 are mounted on the rear of the printed circuit board (PCB)5. The electrical termination may be a passive or an active electricallyresistive device. The passive device is typically an electricallyresistive device having a resistance of 50 Ω. The active device ispreferably an FET input high frequency preamplifier with a 50 Ω outputimpedance, although other active termination devices could alternativelybe used. For optimum operation, the distance between the electricaltermination and the detection device is kept relatively short (typicallyless than 5 mm).

In this device, the upper-most metal electrode 1 is optically opaque toelectromagnetic radiation at the minimum thickness required foreffective conductivity and so any unreflected electromagnetic radiationwill not be transmitted, but will be absorbed by the metal electrode 1.This means that the device provides a particularly effective and idealsolution for the detection of longer wavelengths of electromagneticradiation (for example infra-red radiation). In prior arrangements,radiation transmissive materials which are usually placed between amirror and a detector may lack mechanical robustness, or may haveabsorption bands of their own, thereby limiting the overalleffectiveness of such arrangements for the detection of longerwavelengths. Use of the reflective layer in this device is an efficientand optically simple method for enabling measurement of unreflectedincident energy of the electromagnetic radiation.

A principal application of the described device is that of a combinedreflector and detector for use in multi-pass gas molecular spectroscopy,such as Cavity Ring Down Spectroscopy, for which it is particularly wellsuited, both in terms of its ease of use and simplicity. Otherapplications of the device include use as an inline beam monitor or alaser cavity monitor.

The useful wavelength range of the device is the same as thereflectivity characteristic of the upper-most metal electrode 1 and canextend from soft x-ray/deep ultra-violet (DUV) through the visible, intothe infra-red and right up to the far infra-red. This is a wavelengthrange from 0.15 μm to 1 cm. Furthermore, the reflectivity ratio of theupper-most metal electrode 1 can be modified to give a desired ratio ofreflectivity to absorption characteristic. This modification of thereflectivity ratio is particularly beneficial for Cavity Ring DownSpectroscopy to obtain the optimal output sensitivity and maximumoptical path length for a given application.

It will be appreciated that the device is not restricted to the specificgeometrical configuration described with reference to the Figures. Forexample, whereas the device shown in FIG. 2 has a circular active area,the device could alternatively have a square or rectangular active area.

Similarly, the device has a concave top surface, but other geometricalconfigurations of the top surface are possible. For example, the devicemay have an entirely flat-surface, or a surface with a complex geometry.The shape of the device will ultimately be defined by the limitations ofthe manufacturing process used to deposit the insulating layer 3.

It will be appreciated that the device can also be formed with the firstand/or third electrically conductive layers segmented so that theyprovide a plurality of conductive areas which are electrically isolatedfrom each other. This enables the segmented layer to function as ann-element array (where n is greater than 1). Typically, if the firstlayer is segmented the third layer will be a continuous layer and viceversa.

1. A device for simultaneously reflecting and detecting electromagneticradiation, comprising a first layer made from electrically conductivematerial for simultaneously reflecting and absorbing electromagneticradiation incident at a surface of the layer, wherein said first layersimultaneously separates incident electromagnetic radiation into areflected part and an unreflected part, the first layer being effectiveto reflect the electromagnetic radiation of said reflected part awayfrom the device and to absorb the electromagnetic radiation of theunreflected part, a second layer underlying said first layer, made froma material having an electrical property dependent on an intensity ofelectromagnetic radiation absorbed by said first layer, and a thirdlayer underlying said second layer, made from electrically conductivematerial, wherein said first layer and said third layer form a firstelectrode and a second electrode respectively and electrical voltageand/or current measured between the electrodes is responsive to saidelectrical property and indicative of the intensity of the absorbedelectromagnetic radiation.
 2. A device according to claim 1, whereinsaid surface of said first layer is a specularly reflective surface. 3.A device according to claim 1, wherein said surface of said first layeris a diffusively reflective surface.
 4. A device according to claim 1including a fourth layer positioned in front of said first layer andbeing transparent to incident electromagnetic radiation.
 5. A deviceaccording to claim 4 wherein said fourth layer is transparent to aparticular wavelength range of incident electromagnetic radiation and iseffective as a high, low or band pass filter.
 6. A device according toclaim 4, wherein said fourth layer is at least partially reflective ofelectromagnetic radiation in one or more band of wavelengths whereby toattenuate intensity of electromagnetic radiation incident at saidsurface of said first layer.
 7. A device according to claim 4, whereinsaid fourth layer is a chemically protective layer.
 8. A deviceaccording to claim 4, wherein said fourth layer comprises a plurality oflayers which, in combination, are effective to achieve a desiredadditional optical and/or chemical property.
 9. A device according toclaim 4, wherein said fourth layer conforms to the shape of said firstlayer.
 10. A device as claimed in claim 1, having an electricaltermination for enabling electrical voltage and/or current measurementat high frequency.
 11. A device as claimed in claim 10, wherein saidelectrical termination is a passive electrically resistive element, of50 ohms.
 12. A device as claimed in claim 10, wherein said electricaltermination is an active element with an input impedance ideally matchedto the impedance of the device with an output impedance of 50 ohms. 13.A device as claimed in claim 12, wherein said active element is a FETinput high frequency preamplifier.
 14. A device as claimed in claim 1,wherein the material of said second layer is a piezo-and/orpyro-electrically active material.
 15. A device as claimed in claim 14,wherein the material of said second layer is a piezo-electrically and/orpyro-electrically active polymer.
 16. A device as claimed in claim 15,wherein said material is poly (vinylidene difluoride) (PVDF) or thecopolymer of poly (vinylidene difluoride)/trifluoroethylene (PVDF/TrFE).17. A device as claimed in claim 1 wherein said first, second and thirdlayers are supported in a support surface of an electrically insulatingsubstrate.
 18. A device as claimed in claim 17, wherein said supportsurface has a preformed shape to which said first, second and thirdlayers conform.
 19. A device as claimed in claim 18, wherein saidsupport surface is concave.
 20. A device as claimed in claim 1, whereinsaid third layer has a preformed shape to which said first and secondlayers conform and which supports said first and second layers.
 21. Adevice as claimed in claim 20, wherein said third layer is concave. 22.A device as claimed in claim 17 including a printed circuit board (PCB)mounted on a lower surface of said electrically insulating substrate.23. A device as claimed in claim 20 including a printed circuit board(PCB) mounted on a lower surface of said third layer.
 24. A device asclaimed in claim 22 including electrical circuitry mounted on the PCB.25. A device as claimed in claim 24, wherein said electrical circuitryincludes preamplifier electronics.
 26. A device as claimed in claim 1including screening means for preventing exposure of the device toexternally generated radio frequency interference.
 27. A device asclaimed in claim 26 wherein said screening means comprises a screeningcan having an aperture by which incident electromagnetic radiation canenter the device and reflected electromagnetic radiation can leave thedevice.
 28. A device as claimed in claim 1 wherein said first layer ismade from one or more metal selected from silver, gold, aluminium andcopper.
 29. A device as claimed in claim 1 suitable for detectingelectromagnetic radiation in the wavelength range from 0.15, um to 1.0cm.
 30. A device as claimed in claim 1 wherein said first layer has athickness in the range from 0.5, um to 100, um.
 31. A device as claimedin claim 1 wherein said third layer is segmented to provide a pluralityof conductive areas electrically isolated from each other to provide ann-element array, where n is greater than one, and said first layer is acontinuous metal layer.
 32. A device as claimed in claim 1 wherein saidfirst layer is segmented to provide a plurality of conductive areaselectrically isolated from each other to provide an n-element array,where n is greater than one, and said third layer is a continuous metallayer.
 33. (canceled)